[fpv/spi_host] Fix undecleared variable issue
[fpv/spi_host] Fix undecleared variable issue

There is an assertion that used a varaible declared in the "if"
statement, while the assertion is in the "else" branch.
Please feel free to let me know if you prefer some other approaches to
fix that.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@google.com>

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